Method and apparatus to control a DC-DC converter

ABSTRACT

Control circuits and techniques are provided for use with DC-DC converters. The circuits and techniques may, in some implementations, be used with LED driver systems. In some embodiments control circuits are provided that rely on multiple different feedback paths to adjust a duty cycle of a DC-DC converter. In some embodiments, control circuits are provided that switch between multiple capacitors to provide as duty cycle control signal for use with a DC-DC converter.

FIELD

Subject matter disclosed herein relates generally to electronic circuitsand, more particularly, to control techniques and circuits for use withDC-DC converters.

BACKGROUND

Light emitting diode (LED) driver circuits are often called upon todrive a number of series connected strings of diodes simultaneously. Thestrings of diodes (or “LED channels”) may be operated in parallel, witha common voltage node supplying all of the strings. A DC-DC converter(e.g., a boost converter, a buck converter, etc.) may be employed by theLED driver circuit to maintain a regulated voltage level on the variousLED channels during operation so that all LED channels have adequateoperational power. Feedback from the LED channels may be used to controlthe DC-DC converter. To reduce unnecessary power consumption, it may bedesirable to keep the regulated voltage level on the voltage node to aminimum or near minimum, while still providing adequate power to allchannels.

Some LED driver circuits are only capable of driving LED channels thatare relatively uniform. That is, the driver circuits are only capable ofdriving channels having the same number of LEDs and the same currentlevels. In addition, some driver circuits illuminate all driven LEDsusing the same dimming duty cycle. These operational constraintssimplify the design of the DC-DC converter associated with the LEDdriver circuit. Newer LED driver circuits are being proposed that willallow more complex illumination functionality. For example, someproposed designs may allow different numbers of diodes and differentcurrents to be used in different LED channels. Some designs may alsoallow different dimming duty cycles to be specified for different LEDchannels, in addition, some proposed designs may allow differentillumination phasing in different channels (i.e., the LEDs withindifferent channels may be permitted to turn on at different times).

As will be appreciated, any increase in the functional complexity of LEDdriver circuits, and/or the circuitry they drive, can complicate thedesign of DC-DC converters and/or converter control circuitry for thedrivers. Techniques and circuits are needed that are capable ofproviding DC-DC voltage conversion within LED driver circuits, and/orother similar circuits, that can support this increased complexity.

SUMMARY

In accordance with one aspect of the concepts, systems, circuits, andtechniques described herein, an electronic circuit for use in driving aplurality of loads coupled to a common voltage node, where each load inthe plurality of loads including a series-connected string of loaddevices, comprises: a plurality of current regulators to regulatecurrent through corresponding ones of the plurality of loads; andcontrol circuitry to control a DC-DC converter to generate a regulatedvoltage on the common voltage node, the control circuitry comprising:(a) a duty cycle control unit to control a duty cycle of the DC-DCconverter, the duty cycle control unit being responsive to a duty cyclecontrol signal at a control input thereof that is indicative of a dutycycle to be used by the duty cycle control unit; (b) at least onecapacitor to carry a voltage to act as the duty cycle control signal forthe duty cycle control unit; and (c) at least one error amplifier tofacilitate adjustment of the voltage on the at least one capacitor basedon feedback, the at least one error amplifier being configured togenerate an error signal based, during first time periods, on the outputvoltage of the DC-DC converter and, during second time periods, onfeedback from the plurality of current regulators, wherein the secondtime periods are different from the first time periods.

In accordance with another aspect of the concepts, systems, circuits,and techniques described herein, a method is provided for operating aduty cycle control unit to generate a switching signal for a DC-DCconverter, the DC-De converter to generate an output voltage to power aplurality of light emitting diode (LED) channels coupled to a commonvoltage node, where each LED channel in the plurality of LED channelsincludes a series-connected string of LEDs and the duty cycle controlunit has an input to receive a duty cycle control signal indicative of aduty cycle to be used for the DC-DC converter. More specifically, themethod comprises: generating an error signal for use in adjusting avoltage level on at least one capacitor coupled to the input of the dutycycle control unit based on feedback, wherein generating the errorsignal includes generating the error signal based on the output voltageof the DC-DC converter during first time periods and generating theerror signal based on one or more voltages across one or more currentregulators associated with the plurality of LED channels during secondtime periods, wherein the second time periods are different from thefirst time periods.

In accordance with still another aspect of the concepts, systems,circuits, and techniques described herein, an electronic circuit for usein driving a plurality of loads coupled to a common voltage node, whereeach load in the plurality of loads including a series-connected stringof load devices, comprises: control circuitry for controlling a DC-DCconverter to generate a regulated voltage on the common voltage node,the control circuitry comprising: (a) a duty cycle control unit tocontrol a duty cycle of the DC-DC converter, the duty cycle control unitbeing responsive to a duty cycle control signal at a control inputthereof that is indicative of a duty cycle to be used by the duty cyclecontrol unit; (b) a first capacitor to carry a first voltage to act as aduty cycle control signal for the duty cycle control unit; (c) a secondcapacitor to carry a second voltage to act as a duty cycle controlsignal for the duty cycle control unit; and (d) a switch circuit toalternately couple the first capacitor and the second capacitor to thecontrol input of the duty cycle control unit in response to one or morecontrol signals.

In accordance with a further aspect of the concepts, systems, circuits,and techniques deathbed herein, a method for operating a duty cyclecontrol unit to generate a switching signal for a DC-DC converter, wherethe duty cycle control unit has an input to receive a duty cycle controlsignal indicative of a duty cycle to be used for the DC-DC converter,comprises: alternately coupling at least a first capacitor and a secondcapacitor to the input of the duty cycle control unit, the first andsecond capacitors each having corresponding voltages across them thatact as duty cycle control signals for the duty cycle control unit whenthe corresponding capacitors are coupled to the duty cycle control unit.

In accordance with a still further aspect of the concepts, systems,circuits, and techniques described herein, a control circuit forcontrolling a DC-DC converter to generate a regulated voltage comprises:a duty cycle control unit to control a duty cycle of the DC-DCconverter, the duty cycle control unit being responsive to a duty cyclecontrol signal at a control input thereof that is indicative of a dutycycle to be used by the duty cycle control unit; a first capacitor tocarry a first voltage to act as a duty cycle control signal for the dutycycle control unit; a second capacitor to carry a second voltage to actas a duty cycle control signal for the duty cycle control unit; and aswitch circuit to alternately couple the first capacitor and the secondcapacitor to the control input of the duty cycle control unit inresponse to one or more control signals.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features may be more fully understood from the followingdescription of the drawings in which:

FIG. 1 is a schematic diagram illustrating an exemplary system for usein driving light emitting diodes (LEDs), or other similar load devices,in accordance with an embodiment;

FIG. 2 is a schematic diagram illustrating exemplary boost controlcircuitry that may be used in an LED driver circuit in accordance withan embodiment;

FIGS. 3A and 3B are schematic diagrams illustrating exemplary portionsof a boost control circuit that may be used in an LED driver circuitthat allows variable phasing of LED channels in accordance with anembodiment;

FIG. 4 is a schematic diagram illustrating an exemplary boost controlcircuit that uses multiple feedback paths and dual COMP capacitors inaccordance with an embodiment;

FIG. 5 is a timing diagram illustrating various exemplary controlwaveforms that may be used in a boost control circuit in accordance withan embodiment; and

FIG. 6 is a flowchart illustrating a method for operating a DC-DCconverter in accordance with an embodiment.

DETAILED DESCRIPTION

FIG. 1 is a schematic diagram illustrating an exemplary system 10 foruse in driving light emitting diodes (LEDs), or other similar loaddevices, in accordance with an embodiment. As shown, system 10 mayinclude LED driver circuitry 12 and a boost converter 14. The system 10may drive a plurality of LEDs 16. As shown, the plurality of LEDs 16 maybe arranged in individual, series-connected strings 16 a, . . . , 16 nthat are each coupled to a common voltage node 20. Theseseries-connected strings will be referred to herein as LED channels 16a, . . . , 16 n. Any number of LED channels 16 a, . . . , 16 n may bedriven by system 10 in various embodiments. In addition, in someimplementations, each LED channel 16 a, . . . , 16 n may be allowed tohave a different number of LEDs. The LEDs 16 may be intended to provideany of a number of different illumination functions (e.g., backlightingthr a liquid crystal display, LED panel lighting, LED display lighting,and/or others).

In some embodiments, LED driver circuitry 12 may be implemented as anintegrated circuit (IC) and boost converter 14 may be connectedexternally to the IC. In other embodiments, an IC may be provided thatincludes both LED driver circuitry 12 and boost converter 14. In stillother embodiments, system 10 may be realized using discrete circuitry.As will be appreciated, any combination of integrated circuitry anddiscrete circuitry may be used for system 10 in various implementations.In the discussion that follows, it will be assumed that LED drivercircuitry 12 is implemented as an IC.

Boost converter 14 is a DC-De voltage converter that is used to converta direct current (DC) input voltage V_(IN) to a regulated output voltageon output voltage node 20 for use in driving LEDs 16. As is well known,a boost converter is a form of switching regulator that utilizesswitching techniques and energy storage elements to generate a desiredoutput voltage. Control circuitry for boost converter 14 may be providedwithin LED driver circuitry 12. Although illustrated as a boostconverter in FIG. 1, it should be appreciated that other types of DC-DCconverters may be used in other embodiments (e.g., buck converters,boost-buck converters, etc.).

As illustrated in FIG. 1, LED driver circuitry 12 may include boostcontrol circuitry 22 for use in controlling the operation of boostconverter 14. LED driver circuitry 12 may also include LED dimming logic24 and a number of current sinks 26 a, . . . , 26 n. The current sinks26 a, . . . , 26 n are current regulators that may be use to draw aregulated amount of current through the LED channels 16 a, . . . , 16 nduring LED drive operations. In at least one embodiment, one currentsink 26 a, . . . , 26 n may be provided for each LED channel 16 a, . . ., 16 n. LED dimming logic 24 is operative for controlling the brightnessof the LEDs in the various channels 16 a, . . . , 16 n. LED dimminglogic 24 may control the brightness of an LED channel by, for example,changing the current and/or the pulse width modulation (PWM) duty cycle(or “dimming” duty cycle) of the channel. In some embodiments, LEDdimming logic 24 may be capable of independently controlling both thecurrent level and the dimming duty cycle of each of the LED channels 16a, . . . , 16 n by providing appropriate control signals tocorresponding current sinks 26 a, . . . , 26 n. In some embodiments, LEDdimming logic 24 may also be capable of independently adjusting theillumination “turn on” time or phase of the LED channels 16 a, . . . ,16 n (i.e., the time when a channel first lights up during a cycle).

In at least one embodiment, LED driver circuitry 12 may be userprogrammable. That is, LED driver circuitry 12 may allow a user to setvarious operational characteristics of system 10. One or more datastorage locations may be provided within LIED driver circuitry 12 tostore user-provided configuration information to set operationalparameters such as, for example, dimming duty cycle of different LEDchannels, current levels of different LED channels, illumination “turnon” times of different LED channels, and/or other parameters. Any typeof data storage structures may be used including, for example, flashmemory, RAM memory, EEPROMs, and/or others. In some implementations,off-chip storage may be used for user configuration information. Usingthis approach, one or more pins, terminals, contacts, or leads may beprovided on an IC for use in interfacing with external storage duringdriver operation. An input/output protocol may also be implementedwithin the IC to control the storage interface. In some implementations,a user may also be able to specify which LED channels are active (i.e.,enabled) and which LED channels are inactive disabled). Default valuesmay be used for the different parameters in the absence of user providedvalues.

As described above, boost converter 14 is operative for converting a DCinput voltage V_(IN) into a DC output voltage V_(OUT) that is adequateto supply LED channels 16 a, . . . , 16 n. In the illustratedembodiment, boost converter 14 includes an inductor 30, a diode 32, anda capacitor 34. Other boost converter architectures may alternatively beused. The operating principles of boost converters are well known in theart. To operate properly, a switching signal having appropriatecharacteristics must be provided to boost converter 14. Boost controlcircuitry 22 of LED driver circuitry 12 is operative for providing thisswitching signal. As will be described in greater detail, boost controlcircuitry 22 may draw current from switching node 36 of boost converter14 at a controlled duty cycle to regulate the output voltage V_(out) ina desired manner.

Boost converter 14 and boost control circuitry 22 are operative forproviding an adequate voltage level on voltage node 20 to supportoperation of all active LED channels 16 a, . . . , 16 n. To conserveenergy, however, it may be desired that the voltage level on voltagenode 20 be no higher (or only slightly higher) than a minimum levelrequired to support operation. To achieve this, boost control circuitry22 may rely, at least in part, on feedback from LED channels 16 a, . . ., 16 n. Typically, the voltage level required for a particular LEDchannel will be dictated by the needs of the current sink 26 a . . . ,26 n associated with the channel. That is, each current sink 26 a, . . ., 26 n may require a minimal amount of voltage (e.g., an LEDs regulationvoltage) to support operation for the corresponding LED channel.

In general, the voltage level on each current sink 26 a, . . . , 26 nwill be equal to the difference between the voltage on voltage node 20and the voltage drop across the LEDs in the corresponding LED channel 16a, . . . , 16 n. Because each LED channel 16 a, . . . , 16 n may have adifferent number of LEDs and a different DC current, different LEDchannels may require different minimum voltage levels for properoperation. The LED channel that requires the highest voltage level onnode 20 for proper operation will be referred to herein as the“dominant” LED channel. As will be appreciated, in some implementations,the dominant LED channel may change with time. The dominant channel willoften be the channel that has the most LEDs. If there are multiplechannels having the “most” LEDs, than one of the channels may beselected as the dominant channel based on a selection criterion.

As shown in FIG. 1, in some implementations, optional ballast resistors40 a, . . . , 40 n may be used in one or more of the LED channels 16 a,. . . , 16 n to provide balance between the voltage levels on thevarious current sinks 26 a, . . . , 26 n. As described above, when noballast resistor is present, the voltage across a current sink willtypically be equal to the difference between the boost output voltage onnode 20 and the voltage drop across the LEDs in the correspondingchannel. Ballast resistors 40 a, . . . , 40 n may be provided, forexample, to generate an additional voltage drop in some channels toachieve similar voltages on the various current sinks 26 a, . . . , 26n. In this manner, some of the power dissipation that might haveoccurred on chip within LED driver circuitry 12 can be moved off chip tothe ballast resistors 40 a, . . . , 40 n.

FIG. 2 is a schematic diagram illustrating exemplary boost controlcircuitry 50 in accordance with an embodiment. As will be described ingreater detail, boost control circuitry 50 of FIG. 2 is specially suitedfor use in LED driver systems that do not allow independent control ofillumination “turn on” time (or phase) of different LED channels. Thatis boost control circuitry 50 is best utilized in systems that turn allenabled LED channels on at the same time (although different LEDchannels may use different dimming duty cycles). Boost control circuitry50 may be used within system 10 of FIG. 1 (e.g., as boost controlcircuitry 22) and/or in other systems. In the discussion that follows,boost control circuitry 50 will be described in the context of system 10of FIG. 1.

As shown in FIG. 2 boost control circuitry 50 may include: a first erroramplifier 52; a second error amplifier 54; a boost duty cycle controlunit 56; an inverter 58; a COMP capacitor 60; first, second, and thirdswitches 62, 64, 66; and a sample capacitor 68. Boost duty cycle controlunit 56 is operative for generating a switching signal to be applied ata switching node 70 of a corresponding boost converter (e.g., SW node 36in boost converter 14 of FIG. 1). During system operation, boost dutycycle control unit 56 may draw current from switching node 70 at acontrolled duty cycle in a manner that results in a desired DC voltagelevel at the boost output (e.g., voltage node 20 in FIG. 1). Boost dutycycle control unit 56 may include an input 72 to receive a duty cyclecontrol signal to set a duty cycle of the boost converter. In theillustrated embodiment, the voltage across COMP capacitor 60 coupled toinput 72 of boost duty cycle control unit 56 serves as the duty cyclecontrol signal. Although illustrated as a single capacitor, it should beunderstood that multiple capacitor combinations may be used to performthis function.

First and second error amplifiers 52, 54 are operative for adjusting thevoltage across COMP capacitor 60 using error signals. As describedabove, boost control circuitry 50 of FIG. 2 is specially suited for usein LED driver systems that do not allow independent control ofillumination “on” time. Therefore, during each illumination cycle, therewill be a period during which all of the LED channels are on (i.e.,illuminated). First error amplifier 52 may be used to set the voltageacross COMP capacitor 60 during the periods when all. LED channels areon. During periods when less than all of the LED channels in the systemare on, second error amplifier 54 may be used to set the voltage acrossCOMP capacitor 60. As described previously, in some embodiments, one ormore of the LED channels within a system may be controllably disabled bythe system (based on, for example, user input or some other reason). Insuch embodiments, phrases such as “all LED channels” and “less than allLED channels” used herein are referring to all or less than all“enabled” LED channels.

First and second switches 62, 64 are operative for controllably couplingoutputs 76, 78 of first and second error amplifiers 52, 54 to COMPcapacitor 60 at appropriate times. Control signal 74 (i.e., ALLON) mayhave a first value (e.g., logic one) when all LED channels are on and asecond value (e.g., logic zero) when less than all LED channels are on(i.e., one or more LED channels are off). In the illustrated embodiment,control signal 74 is used to control first switch 62 and an invertedversion of control signal 74 is used to control second switch 64. Thus,the output of first error amplifier 52 will be coupled to COMP capacitor60 when all LED channels are on and the output of second error amplifier54 will be coupled to COMP capacitor 60 when less than all LED channelsare on. As will be appreciated, other switching schemes may be used inother embodiments (e.g., a single switch that couples either output 76or output 78 to comp cap 60 based on control signal 74 without the needfor inverter 58, etc.).

In the illustrated embodiment, first and second error amplifiers 52, 54are trans-conductance amplifiers that each generate an output currenterror signal that is proportional in magnitude to a difference betweentwo corresponding input voltages. When coupled to COMP capacitor 60,these error currents will act to adjust the voltage across the capacitorin a controlled manner. Other types of error amplifiers may be used inother embodiments. As shown in FIG. 2, first error amplifier 52 has anLED regulation voltage VLED_Reg) coupled to a non-inverting inputthereof and voltage feedback signals from LED channels (i.e., VLED<1:n>)coupled to an inverting input thereof. The LED regulation voltage mayrepresent a minimum voltage level required on a current sink (e.g.,current sinks 26 a, . . . 26 n of FIG. 1) to ensure proper operation.The voltage feedback signals may represent voltages across the currentsinks of the driver circuitry (e.g., the voltages on the LED pins 42 a,. . . , 42 n of FIG. 1).

First error amplifier 52 may generate its output error signal based on adifference between a feedback voltage and the regulation voltage. Insome implementations, the feedback voltage that is used may beassociated with the present dominant LED channel. In otherimplementations, an average or mean of the feedback signals of allchannels (or some other combination) may be used. Therefore, duringperiods when all LED channels are on, the voltage on COMP capacitor 60will be adjusted to ensure that the voltage level on all LED pins equalsor exceeds the LED regulation voltage.

During periods when less than all of the LED channels are on, boostcontrol circuitry 50 will simply maintain the voltage on COMP capacitor60 at the level it had when all channels were on. This may be achievedusing second error amplifier 54. As illustrated in FIG. 2, a samplecapacitor 68 may be coupled to a non-inverting input of second erroramplifier 54 and the present boost output voltage (VOUT) may be coupledto the inverting input. As shown, sample capacitor 68 may becontrollably coupled to the boost output VOUT through switch 66, whichis controlled by control signal 74. Thus, when all LED channels are on,switch 66 is closed and the output voltage of the boost converterappears across sample capacitor 68. When one or more LED channels areturned off; third switch 66 is opened and the voltage on capacitor 68 isheld at its present value. Second error amplifier 54 will thereaftergenerate an output error signal based on a difference between thepresent output voltage of the boost converter and the previous valuewhen all channels were on. The resulting error current is coupled toCOMP capacitor 60 through second switch 64 to adjust the voltagethereon. The boost control circuitry 50 of FIG. 2 can be used to controlthe boost converter associated with LED driver circuitry even inimplementations where the dimming duty cycles of the LED channels and/orthe current levels of the LED channels are independently controllable.The boost control circuitry 50 can also be used in implementations wherethe LED channels being driven have different numbers of LEDs.

In some embodiments, the illumination turn on time or phase of thevarious LED channels may be independently controlled. In theseembodiments, there may not always be a period during which all LEDchannels are concurrently illuminated and boost control circuitry 50 ofFIG. 2 may be of limited effectiveness. FIGS. 3A and 3B are schematicdiagrams illustrating exemplary portions of a boost control circuit thatmay be used when variable phasing of LED channels is permitted. FIG. 3Aillustrates a control circuit $0 that may be provided for each LEDchannel in an LED driver system to provide a feedback signal for thechannel. FIG. 3B illustrates as control circuit 110 that may be used toprocess the feedback signals associated with the various channels togenerate a switching signal for a corresponding boost converter. Theboost control circuit of FIGS. 3A and 3B may be used in system 10 ofFIG. 1 (e.g., as boost control circuitry 22) and/or in other systems.

As illustrated in FIG. 3A, control circuit 80 may include: an erroramplifier 82; an inverter 84; a sample capacitor 86; and first, second,third, and fourth switches 88, 90, 92, 94. As described above, in someimplementations, a control circuit 80 may be provided for each LEDchannel in an LED driver system, in the discussion that follows, controlcircuitry 80 of FIG. 3A will be described in the context of a first LEDchannel (LED 1). Error amplifier 82 is operative for generating afeedback signal for the first LED channel based on a difference betweentwo input voltages. In addition, the sources of the two input voltagesof error amplifier 62 may vary during system operation based on, forexample, a dimming duty cycle of the first LED channel. In theillustrated embodiment, switches 88, 90, 92, 94 may be used to changethe inputs applied to error amplifier 82 based on, for example, a pulsewidth modulation (PWM) signal 96 associated with the first LED channel.When PWM signal 96 has a first value (e.g., logic one) corresponding toan “on” portion of the dimming duty cycle of the first LED channel,switches 88 and 90 may be closed and switches 92 and 94 may be open.During this time, an LED regulation voltage may be applied to anon-inverting input of error amplifier 82 and an LED pin voltage VLED<1>(or other feedback) associated with the first LED channel may be appliedto the inverting input of error amplifier 82.

When PWM signal 96 has a second value (e.g., logic zero) correspondingto an “off” portion of the dimming duty cycle of the first LED channel,switches 88 and 90 may be open and switches 92 and 94 may be closed.During this time, a voltage across sample capacitor 86 may be applied tothe non-inverting input of error amplifier 82 and a present outputvoltage of the boost converter (VOUT) may be applied to the invertinginput of error amplifier 82. As shown, sample capacitor 98 may becoupled to the boost output through switch 98, which is controlled byPWM signal 96. Thus, when the first LED channel is on, switch 98 isclosed and the output voltage of the boost converter appears acrosssample capacitor 86. When the first LED channel is turned off, switch 98is opened and the voltage on capacitor 86 is held at its present value.Thus, when the first LED channel is tuned off, error amplifier 82 willgenerate an error signal based on the difference between the presentboost output voltage and the prior boost output voltage when the firstLED channel was on.

As illustrated in FIG. 3B, control circuit 110 may include: an erroramplifier 112, a COMP capacitor 114, and a boost duty cycle control unit120. As in the previously described embodiment, boost duty cycle controlunit 120 is operative for generating a switching signal to be applied ata switching node 122 of a corresponding boost converter (e.g., SW node36 in boost converter 14 of FIG. 1). The voltage across COMP capacitor114 serves as a duty cycle control signal for boost duty cycle controlunit 120. Error amplifier 112 generates an error signal at an outputthereof that adjusts the voltage across COMP capacitor 114 based on, forexample, a difference between two input voltages. In the illustratedembodiment, error amplifier 112 is a trans-conductance amplifier thatgenerates an output current signal (although other types of amplifiersmay be used in other implementations).

In at least one implementation, the voltage applied to the non-invertinginput of error amplifier 112 is an average or mean of the feedbackvoltages of all of the LED channels (i.e., an average of the outputs ofcontroller 80 of each channel or VFB). In some embodiments, the feedbackvoltages of all of the LED channels may be applied to error amplifier112 and the averaging (or other processing) may be performed internal toamplifier 112. In at least one implementation, the control circuits 80of all of the LED channels will be providing feedback all of the time toerror amplifier 112. A reference voltage (VREF) may be applied to theinverting input of error amplifier 112. When one or more of the LEDchannels needs a higher voltage, the value of VFB will be greater thanthe reference voltage and error amplifier 112 will increase the voltageon. COMP capacitor 114. When the LED channels are being overdriven, thevalue of VFB will be less than the reference voltage and error amplifier112 will reduce the voltage on COMP capacitor 114. In either case, boostduty cycle control unit 120 will change the duty cycle of the boostconverter accordingly. The boost control circuit of FIGS. 3A and 3B mayallow a corresponding boost converter to regulate an LED pin for minimumdimming duty cycle without generating significant ripple at the boostoutput.

FIG. 4 is a schematic diagram illustrating an exemplary boost controlcircuit 130 that uses multiple feedback paths and dual COMP capacitorsin accordance with an embodiment. As will be described in greaterdetail, boost control circuit 130 of FIG. 4 is designed to maintain theoutput of the boost converter at a level required by a dominant LEDchannel, while also allowing the boost duty cycle to be rapidly changedwith a sudden change in load. The boost control circuit 130 of FIG. 4may be used in system 10 of FIG. 1 (e.g., as boost control circuitry 22)and/or in other systems.

As illustrated in FIG. 4, boost control circuit 130 may include: first,second, and third error amplifiers 132, 134, 136; a boost duty cyclecontrol unit 138; first and second COMP capacitors 140, 142; an inverter146; a sample capacitor 148; a number of switches 150, 152, 154, 156,158, 160, 162, 166, 168 and a unity gain buffer 164. As in thepreviously described embodiments, boost duty cycle control unit 138 isoperative for generating a switching signal to be applied at a switchingnode 170 of a corresponding boost converter. A duty cycle control signalis applied to an input 174 of boost duty cycle control unit 138 to set aduty cycle of the boost converter. Unlike the previous embodiments,however, the duty cycle control signal is not derived from a single COMPcapacitor, but from two COMP capacitors 140, 142 that are alternatelycoupled to an active COMP node 172 during, for example, successive oddand even PWM dimming cycles. In the illustrated embodiment, switches156, 158 are used to alternately couple the two COMP capacitors 140, 142to active COMP node 172. The operation of all switches 150, 152, 154,156, 158, 160, 162, 166, 168 in the illustrated embodiment will bedescribed in detail below.

When first COMP capacitor 140 is coupled to active COMP node 172, secondCOMP capacitor 142 is used to sample the maximum voltage on lint COMPcapacitor 140 the voltage corresponding to the “on” period of thedominant LED channel) for use during the next PWM cycle, and vice versa.When the next PWM cycle starts, the second COMP capacitor 142 is coupledto active COMP node 172 and first COMP capacitor 140 is decoupled fromnode 172. Because second COMP capacitor 142 sampled the highest voltageacross the first COMP capacitor from the previous PWM cycle, the boostduty cycle control unit 138 can adjust almost instantaneously to thecorrect duty cycle for the dominant LED channel. Because of this rapidadjustment, control circuit 130 is capable of supporting very highdimming ratios (i.e., dimming duty cycles that generate a large amountof dimming, with very short “on” periods) without negatively effectingsystem stability.

Third error amplifier 136 is operative for generating an error signal atan output thereof to adjust the voltage across the COMP capacitor thatis currently coupled to active COMP node 172. The error signal isgenerated based on a difference between two input signals. Based on thecurrent state of the dominant LED channel (i.e., on or off), thenon-inverting input of error amplifier 136 will be received from eitherfirst error amplifier 132 or second error amplifier 134 (eachcorresponding to a different feedback path from the LED channels). Theinverting input of third error amplifier 136 may be coupled to a fixedreference voltage (e.g. 12 volts in the illustrated embodiment). Thefixed reference voltage used with third error amplifier 136 may be thecommon mode voltage for the differential input, if the first and seconderror amplifiers 132, 134 do not have errors, then the outputs of bothamplifiers may be the same as reference voltage. In this regard, theabsolute do value of the fixed reference voltage can vary fromimplementation to implementation. In the illustrated embodiment, thirderror amplifier 136 comprises a trans-conductance amplifier and firstand second error amplifiers 132, 134 comprise differential amplifiers,although other types of amplifiers can be used in other implementations.

When the dominant LED channel is on, switch 150 is closed (and switch152 is open) and the output of first error amplifier 132 is coupled tonon-inverting input of error amplifier 136. During this time period, theboost output voltage be regulated to the highest level required by thecorresponding system. First error amplifier 132 generates an outputerror signal based on a voltage difference between feedback from the LEDchannels (e.g., one or more LED pin voltages, VFB_LED<6:1>) and areference voltage (VREF) (e.g., the LED pin regulation voltage). In atleast one embodiment, first error amplifier 132 uses the feedbackvoltage of the dominant LED channel to generate the output error signal.In other embodiments, other feedback signals may be used (e.g., anaverage or mean of all LED pin voltages, etc).

When the dominant LED channel is off switch 152 is closed (and switch150 is open) and the output of second error amplifier 134 is coupled tothe non-inverting input of error amplifier 136. Second error amplifier134 generates an output error signal based on a voltage differencebetween a current value of the boost output voltage and the value of theboost output voltage during the most recent PWM on period of thedominant channel. Switch 154 will be closed during the on portion of thediming duty cycle of the dominant channel, allowing sample capacitor 148to sample the corresponding boost output voltage. When the off period ofthe dimming duty cycle of the dominant channel starts, switch 154 opensand the voltage on sample capacitor 148 is held at its current value.The action of second error amplifier 134 is designed to maintain theoutput voltage of the boost converter at the value it had during the onportion of the dimming duty cycle of the dominant LED channel.

As described above, active COME node 172 may be alternately switchedbetween first and second COMP capacitors 140, 142 during odd and evenPWM cycles. In the illustrated embodiment, switches 156 and 158 are usedto effect this result. During odd cycles, switch 156 will be closed andswitch 158 will be open. During even cycles, switch 158 will be closedand switch 156 will be open. When first COMP capacitor 140 is coupled toactive COMP node 172, second COMP capacitor 142 may be used to samplethe maximum voltage on first COMP capacitor 140. In the illustratedembodiment, this sampling is realized using unity gain buffer 164 andswitches 160, 162, 166, 168. Unity gain buffer 164 may be used to chargethe capacitor that is currently sampling. During odd PWM cycles, switch162 will be closed coupling unity gain buffer 164 to second COMPcapacitor 142. During even PWM cycles, switch 160 will be closedcoupling unity gain buffer 164 to first COMP capacitor 140.

As shown in FIG. 4, switch 166 is coupled between first COMP capacitor140 and an input of unity gain buffer 164. Likewise, switch 168 iscoupled between second COMP capacitor 142 and the input of unity gainbuffer 164. Switch 166 is controlled by a signal S1sub that closes theswitch during the “on” period of the dominant LED channel in odd PWMcycles (and opens switch 166 otherwise). When switch 166 is closed, thevoltage across first COMP capacitor 140 is applied to the input of unitygain buffer 164. The input node of unity gain buffer 164 behaves as aholding node that holds the voltage level applied thereto, even afterswitch 166 subsequently opens. Based on the voltage at the input node,unity gain buffer 164 sets the voltage of second COMP capacitor 142 toequal the voltage that was on first COMP capacitor 140 dining the mostrecent “on” period of the dominant LED channel (i.e., the maximum valueof the voltage on first capacitor 140 during the odd PWM cycle). Becausethe input node of unity gain buffer 164 behaves as a holding node,second COMP capacitor 142 does not have to charge up to the maximumvoltage value of first COMP capacitor 140 within the limited time periodthat switch 166 is closed (which can be very short when high dimmingratios are used by the dominant channel). Instead, second COMP capacitor142 can continue to charge even after switch 166 is closed, until itreadies the maximum voltage value of first COMP capacitor 140.

In a similar fashion, switch 168 is controlled by a signal S2sub thatcloses the switch during the “on” period of the dominant LED channel ineven PWM cycles (and opens switch 168 otherwise). When switch 168 isclosed, the voltage across second COMP capacitor 142 is applied to theinput of unity gain buffer 164 which then sets the voltage of first COMPcapacitor 140 accordingly. Once again, the input node unity gain buffer164 will act as a holding node so that first COMP capacitor 140 cancontinue to charge even after switch 168 opens. In some embodiments, theinput node of unity gain buffer 164 may not act as a holding node. Forexample, in some implementations, dimming ratios may only be used thatwill allow the appropriate capacitor to be fully charged during the “on”period of the dominant LED channel.

As described previously, because first COMP capacitor 140 and secondCOMP capacitor 142 each sample the highest voltage of the othercapacitor, when they are subsequently connected to active COMP node 172,they immediately apply the correct voltage to the node 172 for thedominant LED channel. This allows boost duty cycle control unit 138 torapidly adjust the duty cycle of the boost converter to the necessaryvalue. Because of this rapid adjustment, control circuit 130 is capableof supporting very high dimming ratios without negatively effectingsystem stability.

In the embodiments described above, various switching arrangements areshown for use in swathing components into and out of an active circuit.It should be appreciated that many alternative switching schemes may beused to achieve these switching functions and the particular switchingschemes shown are merely illustrative of example arrangements. Forexample, as described above, switches 156 and 158 of FIG. 4 may beswitched in a manner that alternately couples COMP capacitor 140 andCOMP capacitor 142 to active COMP node 172. Instead of using two singlepole, single throw (SPST) switches as illustrated, one single pole,double throw (SPDT) switch (or some other switching arrangement) may beused to achieve substantially the same function. Similar substitutionsmay be used in other circuits described herein. Any types of switchesmay be used in various embodiments. In some embodiments, for example,the switches may include semiconductor based switches implemented on oroff chip using transistors (e.g., field effect transistors (FETS),bipolar junction transistors (BJTs), etc.) or other semiconductorswitching devices. In an alternative approach, electro-mechanicalswitches implemented off-chip or on a secondary chip of a multi-chipmodule may be used. A combination of semiconductor based switches andelectro-mechanical switches may also be used in some embodiments.

FIG. 5 is a timing diagram illustrating various exemplary waveforms thatmay be used as control signals to control switches in boost controlcircuit 130 of FIG. 4 in accordance with an embodiment. Waveform 180(PWM_(DDM)) represents a possible dimming duty cycle signal for thedominant channel in an LED driver system. As shown in FIG. 4, thissignal may be used to control switch 150 and switch 154. Waveform 182 (PWM_(DOM) ) represents an inverse of waveform 180. As shown in FIG. 4,this signal may be used to control switch 152. Waveform 184 representscontrol, signal S1 that may be used to identify odd PWM cycles in an LEDdriver system. With reference to FIG. 4, this signal may be used tocontrol switch 156 and switch 162 in the illustrated embodiment.Waveform 186 represents control signal S2 that may be used to identifyeven PWM cycles in the LED driver system. With reference to FIG. 4, thissignal may be used to control switch 158 and switch 160 in anembodiment.

Waveform 188 (S1 _(sub)) represents an “on” portion of a duty cycle ofthe dominant LED channel of the LED driver system during the odd PWMcycles. As shown in FIG. 5, waveform 188 only includes “on” pulsesduring corresponding pulses in waveform 184 (but not during pulses ofwaveform 186). With reference to FIG. 4, this signal may be used tocontrol switch 166 in an embodiment. Waveform 190 (S2 _(sub)) representsan “on” portion of a duty cycle of the dominant LED channel of the LEDdriver system during the even PWM cycles. As shown in FIG. 5, waveform190 only includes “on” pulses during corresponding pulses in waveform186 (but not during pulses of waveform 184). With reference to FIG. 4,this signal may be used to control switch 168 in an embodiment.

FIG. 6 is a flowchart illustrating a method 200 for operating a DC-DCconverter in accordance with an embodiment. The method 200 may be usedto control DC-DC converters associated with LED driver circuits and/orother types of circuits and systems. A duty cycle control unit may beprovided to generate a switching signal for use by the DC-DC converterto set an output voltage thereof. The duty cycle control unit may havean input to receive a duty cycle control signal that is indicative ofthe duty cycle to be used. First and second capacitors may be providedto carry voltage levels to be used as the duty cycle control signal. Thefirst and second capacitors may be alternately coupled to the input ofthe duty cycle control circuit to provide the duty cycle control signal(block 202). In some implementations, the first capacitor may be coupledto the input of the duty cycle control circuit during, for example, oddPWM dimming cycles and the second capacitor may be coupled to the inputof the duty cycle control circuit during even PWM dimming cycles. Thesecond capacitor may be used to sample the maximum voltage on the firstcapacitor during periods when the first capacitor is coupled to the dutycycle control unit (block 204). Likewise, the first capacitor may beused to sample the maximum voltage on the second capacitor duringperiods when the second capacitor is coupled to the duty cycle controlunit (block 206). In this manner, when the capacitors are switched, thenew capacitor will always have the correct voltage value for use withthe dominant LED channel. The maximum voltage on each capacitor may bethe voltage that exists on the capacitor when the dominant channel isilluminated. The voltage of the capacitor that is currently coupled tothe duty cycle control circuit will be adjusted based on feedback. Asdescribed above, in some implementations, two different feedback pathsmay be used to adjust the voltage on the capacitor, in some embodiments,three or more capacitors may be used in block 202. In addition, asdescribed previously, a multi-capacitor combination may be used to storea single voltage in some implementations.

As described above, in some implementations, the dominant LED channelmay change with time. For example, in some implementations, a user maybe permitted to disable one or more LED channels during systemoperation. If one of the disabled channels is the current dominantchannel, a new dominant channel needs to be identified. In someimplementations, it may be possible to add one or more LEDs to a channelafter system deployment. This can also affect the dominant LED channel.In addition, during system operation, it may be discovered that one ormore of the non-dominant LED channels is not receiving enough power. Inthis case, the underpowered channel may be made the dominant channel. Insome embodiments, one or more components or controllers may be providedwithin LED driver circuitry for identifying and tracking a dominant LEDchannel. As used herein, the term “controller” is meant to include bothdigital and analog controllers and may include, for example,programmable or reconfigurable processors, embedded processors, ASICs,and/or digital or analog circuits. Controllers may be implemented eitheron or off chip in different embodiments.

Referring back to FIG. 1, in some implementations, a priority queue 38may be maintained that tracks the various LED channels in order ofpriority. A highest priority channel 44 in the queue 38 may representthe dominant LED channel. Digital memory may be provided within LEDdriver circuitry 12 to store priority queue 38. Priority queue 38 may becontinually updated during system operation so that the dominant LEDchannel is always known. Priority queue 38 may provide the updateddominant LED channel information to LED dimming logic 24 and/or boostcontrol circuitry 22. LED dimming logic 24 may need this information toprovide the appropriate dimming duty cycle information to boost controlcircuitry 22 for use in controlling boost converter 14.

In some implementations, a queue manager 46 may be provided formaintaining and updating priority queue 38. Queue manager 46 may, forexample, include a digital or analog controller that is capable ofidentifying the occurrence of certain events and/or conditions that mayrequire a change in LED channel priority. In some implementations, firexample, queue manager 46 may receive feedback from LED channels 16 a, .. . , 16 n. This feedback may include, for example, voltage levels onthe LED pins 42 a, . . . , 42 n of the LED driver circuitry 12, or someother feedback. If queue manager 46 detects, based on the feedback, thatone of the LED channels requires more voltage (e.g., the pin voltage forthe channel is below a specified regulation voltage), it may move thatchannel to the top of priority queue 38. When the LED channel is moved,all of the other channels may be moved down in priority. Queue manager46 may also have access to information describing which LED channelshave been disabled by a user. If the highest priority LED channel in thequeue 38 is disabled, queue manager 46 may move that channel to thelowest priority position in queue 38. Other LED channels may then bemoved up in priority to accommodate the new lowest priority channel. Inone possible approach, the LED channels may initially be listed in adefault order within priority queue 38 (e.g., by channel number, etc.).The action of queue manager 46 may they rearrange and maintain the orderof the channels so that the channel in the highest priority position 44is the dominant LED channel.

In at least one embodiment, instead of a queue, one or more storagelocations may be provided within LED driver circuitry 12 to record andtrack the identity of the current dominant LED channel. A controller maybe provided to continually update the identity of the dominant channelstored in the storage location(s) based on events and conditions. Othertechniques for identifying and tracking a dominant LED channel beingdriven by LED driver circuitry may alternatively be used.

As described previously, in some implementations, LED driver circuitry12 of FIG. 1 may be partially or fully implemented as an IC or as amulti-chip module (MCM). In such embodiments, the various boost controlcircuits described herein (e.g., boost control circuitry 50 of FIG. 2,etc.) may be fully implemented on-chip or one or more elements thereof(e.g., one or more capacitors or other components) may be implementedoff chip. In addition, it should be understood that the elements of theboost control circuitry will not necessarily be located in closeproximity to one another within a realized circuit, which may be anintegrated circuit or a multi-chip circuit in some embodiments. That is,in some implementations, the elements may be spread out within a largersystem and coupled together using appropriate interconnect structures.

In the embodiments described above, features are described in thecontext of LED driver systems that utilize current sinks to draw currentdown through LED channels. It should be appreciated that other types ofcurrent regulation devices may be used in other embodiments. Forexample, in some embodiments, current sources are used that may belocated near the boost output (e.g., near node 20 in FIG. 1) and whichdrive current downward through the LED channels, in these embodiments,feedback signals for use in boost control may be derived based on, forexample, voltage drops across the various current sources. In someembodiments, these current sources may be located external to an LEDdriver integrated circuit (e.g., outside LED driver circuitry 12 of FIG.1, etc.). As will be appreciated, the use of external current sourcescan allow higher maximum currents to be used in some implementations. Insome embodiments, external current sinks may be used. For example, withreference to FIG. 1, in some implementations, the current sinks 26 a-26n may be located outside the LED driver circuitry 12. In each of thesedifferent scenarios, the boost control circuits and techniques describedabove can still be implemented.

In the description above, techniques and circuits for providing controlfor a boost converter or other DC-DC converter have been discussed inthe context of LED drives circuitry. It should be appreciated, however,that these techniques and circuits may also be used in otherapplications. For example, in some implementations, the describedtechniques and circuits may be used in driver circuits that drive loaddevices other than LEDs. The described techniques and circuits may alsohave application in other types of systems, components, and devices thatrequire the generation of a regulated voltage level.

Having described exemplary embodiments of the invention, it will nowbecome apparent to one of ordinary skill in the art that otherembodiments incorporating their concepts may also be used. Theembodiments contained herein should not be limited to disclosedembodiments but rather should be limited only by the spirit and scope ofthe appended claims. All publications and references cited herein areexpressly incorporated herein by reference in their entirety.

What is claimed is:
 1. An electronic circuit for use in driving aplurality of loads coupled to a common voltage node, each load in theplurality of loads including a series-connected string of load devices,the electronic circuit comprising: a plurality of current regulators toregulate current through corresponding ones of the plurality of loads;and control circuitry to control a DC-DC converter to generate aregulated voltage on the common voltage node, the control circuitrycomprising: a duty cycle control unit to control a duty cycle of theDC-DC converter, the duty cycle control unit being responsive to a dutycycle control signal at a control input thereof that is indicative of aduty cycle to be used by the duty cycle control unit; at least onecapacitor to carry a voltage to act as the duty cycle control signal forthe duty cycle control unit; and at least one error amplifier tofacilitate adjustment of the voltage on the at least one capacitor basedon feedback, the at least one error amplifier being configured togenerate an error signal to adjust the voltage on the at least onecapacitor based, during first time periods, on the output voltage of theDC-DC converter and, during second time periods, on feedback from theplurality of current regulators, wherein the second time periods aredifferent from the first time periods.
 2. The electronic circuit ofclaim 1, wherein: the plurality of loads includes a plurality of lightemitting diode (LED) channels, wherein each LED channel in the pluralityof LED channels includes a series-connected string of LEDs.
 3. Theelectronic circuit of claim 2, wherein: the at least one error amplifieris configured to generate the error signal based on a difference betweena present output voltage of the DC-DC converter and a past outputvoltage of the DC-DC converter, during the first time periods.
 4. Theelectronic circuit of claim 3, wherein: the at least one error amplifieris configured to generate the error signal based on a difference betweena voltage across a current regulator associated with a first LED channeland an LED regulation voltage associated with the first LED channel,during the second time periods.
 5. The electronic circuit of claim 4,wherein: the first time periods include “off” portions of a dimming dutycycle of the first LED channel; and the second time periods include “on”portions of the dimming duty cycle of the first LED channel; wherein thepast output voltage of the DC-DC converter is a voltage during a mostrecent “on” portion of the dimming duty cycle of the first LED channel.6. The electronic circuit of claim 4, wherein: the first time periodsinclude periods when all LED channels in the plurality of LED channelsare on; and the second time periods include periods when less than allLED channels in the plurality of LED channels are on; wherein the pastoutput voltage of the DC-DC converter is a voltage during a most recentperiod when all LED channels in the plurality of LED channels were on.7. The electronic circuit of claim 4, wherein: the first LED channelcomprises a dominant LED channel, the dominant LED channel being achannel in the plurality of LED channels that requires a highest voltageon the common voltage node, wherein the dominant LED channel can changewith time.
 8. The electronic circuit of claim 3, wherein: the at leastone error amplifier is configured to generate the error signal based ona difference between an average voltage across multiple currentregulators of the plurality of current regulators and an LED regulationvoltage during the second time periods.
 9. The electronic circuit ofclaim 2, further comprising: LED dimming logic coupled to the pluralityof current sinks, wherein the LED dimming logic is capable ofindividually controlling at least some of the plurality of currentregulators to set different dimming duty cycles and different currentlevels for corresponding LED channels.
 10. The electronic circuit ofclaim 9, wherein: the LED dimming logic is capable of individuallycontrolling at least some of the plurality of current regulators to setdifferent illumination “turn on” times for different LED channels. 11.The electronic circuit of claim 2, wherein: the electronic circuit isimplemented as an integrated circuit having at least one contact forconnection to an external DC-DC converter.
 12. The electronic circuit ofclaim 2, wherein: the at least one capacitor includes at least one firstcapacitor to carry a first voltage to act as the duty cycle controlsignal for the duty cycle control unit; the electronic circuit furthercomprising: at least one second capacitor to carry a second voltage toact as the duty cycle control signal for the duty cycle control unit;and a switch circuit to alternately couple the first capacitor and thesecond capacitor to the control input of the duty cycle control unit inresponse to one or more control signals.
 13. The electronic circuit ofclaim 1, wherein: the plurality of current regulators includes aplurality of current sinks.
 14. A method for operating a duty cyclecontrol unit to generate a switching signal for a DC-DC converter, theDC-DC converter to generate an output voltage to power a plurality oflight emitting diode (LED) channels coupled to a common voltage node,each LED channel in the plurality of LED channels including aseries-connected string of LEDs, the duty cycle control unit having aninput to receive a duty cycle control signal indicative of a duty cycleto be used for the DC-DC converter, the method comprising: generating anerror signal for use in adjusting a voltage level on at least onecapacitor coupled to the input of the duty cycle control unit based onfeedback, wherein generating the error signal includes generating theerror signal based on the output voltage of the DC-DC converter duringfirst time periods and generating the error signal based on one or morevoltages across one or more current regulators associated with theplurality of LED channels during second time periods, wherein the secondtime periods are different from the first time periods.
 15. The methodof claim 14, wherein: the first time periods are “on” portions of adimming duty cycle of a first LED channel; and the second time periodsare “off” portions of the dimming duty cycle of the first LED channel.16. The method of claim 15, wherein: the first LED channel is a dominantLED channel, wherein the dominant LED channel is a channel in theplurality of LED channels that requires a highest voltage on the commonvoltage node.
 17. The method of claim 14, wherein: the first timeperiods are periods during which all of the LED channels in theplurality of LED channels are on; and the second time periods areperiods during which less than all of the LED channels in the pluralityof LED channels are on.
 18. The method of claim 14, wherein: generatingthe error signal based on one or more voltages across one or morecurrent sinks associated with the plurality of LED channels includesgenerating the error signal based on a voltage across a current sinkassociated with a dominant LED channel, wherein the dominant LED channelis a channel in the plurality of LED channels that requires a highestvoltage on the common voltage node.
 19. The method of claim 14, wherein:generating the error signal based on one or more voltages across one ormore current sinks associated with the plurality of LED channelsincludes generating the error signal based on an average of voltagesacross multiple current sinks.
 20. An electronic circuit for use indriving a plurality of loads coupled to a common voltage node, each loadin the plurality of loads including a series-connected string of loaddevices, the electronic circuit comprising: control circuitry forcontrolling a DC-DC converter to generate a regulated voltage on thecommon voltage node, the control circuitry comprising: a duty cyclecontrol unit to control a duty cycle of the DC-DC converter, the dutycycle control unit being responsive to a duty cycle control signal at acontrol input thereof that is indicative of a duty cycle to be used bythe duty cycle control unit; a first capacitor to carry a first voltageto act as a duty cycle control signal for the duty cycle control unit; asecond capacitor to carry a second voltage to act as a duty cyclecontrol signal for the duty cycle control unit; and a switch circuit toalternately couple the first capacitor and the second capacitor to thecontrol input of the duty cycle control unit in response to one or morecontrol signals.
 21. The electronic circuit of claim 20, wherein: theplurality of loads includes a plurality of light emitting diode (LED)channels, wherein each LED channel in the plurality of LED channelsincludes a series-connected string of LEDs.
 22. The electronic circuitof claim 21, wherein: the switch circuit is controlled to couple thefirst capacitor to the control input of the duty cycle control unitduring odd pulse width modulation (PWM) dimming cycles and the secondcapacitor to the control input of the duty cycle control unit duringeven PWM dimming cycles.
 23. The electronic circuit of claim 22,wherein: the switch circuit is a first switch circuit; and the controlcircuitry further comprises a second switch circuit to allow each of thefirst and second capacitors to sample a maximum voltage value of theother of the first and second capacitors when the other of the first andsecond capacitors is coupled to the control input of the duty cyclecontrol unit, in response to one or more control signals.
 24. Theelectronic circuit of claim 23, wherein: the second switch circuitincludes a unity gain buffer to set a voltage of one of the first andsecond capacitors when the other of the first and second capacitors iscoupled to the control input of the duty cycle control unit.
 25. Theelectronic circuit of claim 21, further comprising: an error amplifierto adjust a voltage on a capacitor that is currently coupled to thecontrol input of the duty cycle control unit using an output errorsignal, wherein the capacitor that is currently coupled to the controlinput of the duty cycle control unit is either the first capacitor orthe second capacitor.
 26. The electronic circuit of claim 25, wherein:the error amplifier is configured to generate the output error signalbased on a difference between feedback from at least one LED channel anda reference voltage, during an “on” period of a dimming duty cycle of adominant LED channel; and the error amplifier is configured to generatethe output error signal based on a difference between a current outputof the DC-DC converter and a previous output of the DC-DC converterduring an “off” period of the dimming duty cycle of the dominant LEDchannel; wherein the dominant LED channel is a channel that requires ahighest voltage on the common voltage node.
 27. The electronic circuitof claim 26, wherein: the previous output of the DC-DC converter is anoutput of the DC-DC converter during a most recent “on” period of thedimming duty cycle of the dominant LED channel.
 28. The electroniccircuit of claim 26, wherein: the dominant LED channel can vary withtime; and the electronic circuit further comprises a controller to trackthe identity of the dominant LED channel.
 29. The electronic circuit ofclaim 25, wherein: the error amplifier is a first error amplifier andthe switch circuit is a first switch circuit; and the control circuitryfurther comprises second and third error amplifiers and a second switchcircuit, the second switch circuit to alternately couple the second andthird error amplifiers to a first input of the first error amplifier inresponse to one or more control signals, where a fixed voltage level isapplied to a second input of the first error amplifier.
 30. Theelectronic circuit of claim 29, wherein: the second switch circuit iscontrolled to alternately couple the second and third error amplifiersto the first input of the first error amplifier based on a dimming dutycycle of a dominant LED channel in the plurality of LED channels,wherein the dominant LED channel is a channel that requires a highestvoltage on the common voltage node.
 31. The electronic circuit of claim21, further comprising: a plurality of current sinks to draw currentthrough corresponding ones of the plurality of LED channels; and LEDdimming logic coupled to the plurality of current sinks, wherein the LEDdimming logic is capable of individually controlling at least some ofthe plurality of current sinks to set different dimming duty cycles anddifferent current levels for corresponding LED channels.
 32. Theelectronic circuit of claim 31, wherein: the LED dimming logic iscapable of individually controlling at least some of the plurality ofcurrent sinks to set different illumination turn on times forcorresponding LED channels.
 33. The electronic circuit of claim 21,wherein: the electronic circuit is implemented as an integrated circuithaving at least one contact for connection to an external DC-DCconverter.
 34. A method for operating a duty cycle control unit togenerate a switching signal for a DC-DC converter, the duty cyclecontrol unit having an input to receive a duty cycle control signalindicative of a duty cycle to be used for the DC-DC converter, themethod comprising: alternately coupling at least a first capacitor and asecond capacitor to the input of the duty cycle control unit, the firstand second capacitors each having corresponding voltages across themthat act as duty cycle control signals for the duty cycle control unitwhen the corresponding capacitors are coupled to the duty cycle controlunit.
 35. The method of claim 34, wherein: the output voltage of theDC-DC converter is used to power a plurality of light emitting diode(LED) channels coupled to a common voltage node, each LED channel in theplurality of LED channels including a series-connected string of LEDs,wherein the LED channels are driven in a series of pulse widthmodulation (PWM) dimming cycles; and alternately coupling includescoupling the first capacitor to the input of the duty cycle control unitduring odd PWM dimming cycles and coupling the second capacitor to theinput of the duty cycle control unit during even PWM dimming cycles. 36.The method of claim 35, further comprising: coupling an error signalfrom an error amplifier to the input of the duty cycle control unit toadjust voltages on the first and second capacitors based on feedback,wherein the error signal adjusts the voltage on the first capacitor whenthe first capacitor is coupled to the input of the duty cycle controlunit and the error signal adjusts the voltage on the second capacitorwhen the second capacitor is coupled to the input of the duty cyclecontrol unit.
 37. The method of claim 35, further comprising: sampling amaximum voltage value of the first capacitor using the second capacitorwhen the first capacitor is coupled to the input of the duty cyclecontrol unit; and sampling a maximum voltage value of the secondcapacitor using the first capacitor when the second capacitor is coupledto the input of the duty cycle control unit.
 38. The method of claim 37,wherein: sampling a maximum voltage value of the first capacitorincludes coupling the voltage of the first capacitor to the secondcapacitor during an “on” portion of a dimming duty cycle of a dominantLED channel during odd PWM dimming cycles, wherein the dominant LEDchannel is a channel in the plurality of LED channels that requires ahighest voltage on the common voltage node.
 39. The method of claim 38,wherein: coupling the voltage of the first capacitor to the secondcapacitor includes coupling the voltage through a unity gain buffer. 40.The method of claim 34, wherein: alternately coupling includesalternately coupling a first capacitor, a second capacitor, and a thirdcapacitor to the input of the duty cycle control unit.
 41. A controlcircuit for controlling a DC-DC converter to generate a regulatedvoltage, the control circuit comprising: a duty cycle control unit tocontrol a duty cycle of the DC-DC converter, the duty cycle control unitbeing responsive to a duty cycle control signal at a control inputthereof that is indicative of a duty cycle to be used by the duty cyclecontrol unit; a first capacitor to carry a first voltage to act as aduty cycle control signal for the duty cycle control unit; a secondcapacitor to carry a second voltage to act as a duty cycle controlsignal for the duty cycle control unit; and a switch circuit toalternately couple the first capacitor and the second capacitor to thecontrol input of the duty cycle control unit in response to one or morecontrol signals.